As he arrived in France for the annual meeting of the Group of 7 powers, Mr. Trump posted a message on Twitter citing the International Emergency Economic Powers Act of 1977 — a law meant to enable a president to isolate criminal regimes but not intended to be used to cut off economic ties with a major trading partner because of a disagreement over tariffs…

“In negotiations, and especially in high-stakes negotiations, the side that reacts emotionally generally is the side that does not do well,” he said. “The U.S. side is approaching this from a more emotional side, while China is more calm and calculating.”

Mr. Trump’s threat to invoke the 1977 act to force companies to leave China would be his most recent unorthodox use of authorities that Congress delegated to the presidency for exigent circumstances.

The president previously threatened to use emergency powers to impose tariffs on Mexican goods, unless the Mexican government did more to stop migrants from illegally entering the United States. He backed off after Mexico promised to take tougher action.